The present invention relates to an optical gain equalization unit, an optical gain equalization method, and an optical fiber transmission line, and more particularly to an optical gain equalization unit, an optical gain equalization method, and an optical fiber transmission line for use in an optical fiber transmission line having multiple lines over which multiplexed signal lights are sent.
A conventional optical wavelength multiplexing system have multiplexed optical signals from a plurality of optical signal sources for transmission through an optical fiber transmission line on which Erbium Doped Fiber Amplifiers (EDFA) are installed. At the receiving end, an optical demultiplexer demultiplexes multiplexed light signals into a plurality of channels and sends each to a light receiver.
Normally, gain equalization units are also installed on the optical fiber transmission line on which erbium doped fiber amplifiers are installed. On such a transmission line, one gain equalization unit equalizes the gains of signals over a range of transmission line where a plurality of erbium doped fiber amplifiers are installed; the gain equalization unit is installed so that the gain equalization of each relay range is optimized. This optimization technology is described in Technical Report of Electronic Data Communication, OSC95-35, pp. 13-19, July 1995.
However, an optical fiber transmission line available today includes at least two communication lines, one up line and one down line. And, erbium doped fiber amplifiers are installed on each transmission line at a predetermined interval. Therefore, it is difficult to install gain equalization units at different points on a transmission line. In addition, because of variations in parts and so forth, the number of gain equalizers required for one line may differ from that required by another. This means that, even when the number of required gain equalization units depends on the line, the loss of a predetermined range of the optical fiber transmission line must be equal among lines.